Relationship of myelin internode elongation and growth in the rat sural nerve.

Abstract

AbstractThe lengthening of myelin internodes along peripheral nerve fibers was studied in rat sural nerve during the period of myelination and subsequent growth. Sural nerves were examined from 34 animals ranging in age from 2 to 140 days. Analyses of osmicated, epon‐embedded and transversely sectioned nerves indicated that myclination of rat sural nerve commenced in the two to five day period and continued until day 20. Direct measurements of consecutive myelin internodes along representative teased fibers disclosed a progressive increase of internode length and fiber diameter. A direct relationship was shown between the rate of increment of the largest internodes and the longitudinal growth rate of the rat hind limb as determined by the combined lengths of the disarticulated femur and tibia. Increases of internode lengths among the entire population of myelinated fibers also appeared to correlate with the longitudinal growth pattern of the hind limb. This was assessed by comparing the size distribution of myelinated fibers in adult sural nerve with a theoretical distributional profile based upon the assumption that internode length is solely determined by amount of longitudinal growth after myelination. Assuming that internode elongation is determined by growth, it was shown that the quantity of longitudinal growth after myelination could account for the size and distribution of all myelin internodes in the adult sural nerve. In addition, the relationships between myelin internodes and fiber size at different ages was expressed by slopes of the regression lines. A significant change in this value for sural nerve fibers in the 30–40 day age period indicated that there was an increase of composite fiber diameter relative to internode length during the interval. During the remaining periods of growth, the diameters of myelinated fibers increased at a rate comparable to the increase of internode length.